Electrophotographic (EP) laser printing employs a toner containing pigment components and thermoplastic components for transferring a toner image formed on selected areas of the surface of an insulating, photoconducting material to an image receiver, such as plain paper, coated paper, transparent substrate (conducting or insulative), or an intermediate transfer medium.
There is a demand in the laser printer industry for multi-colored images. The image quality can be enhanced by a large number of approaches, including the technique which utilizes small particle developer including dry toner having an average particle size less than 5 .mu.m; see, e.g., U.S. Pat. Nos. 4,927,727; 4,968,578; 5,037,718; and 5,284,731. However, it has also been known that the electrophotographic dry toner having particle size less than 1 .mu.m is very hard to prepare due to increased specific area, and consequently, liquid toner has become one of the solutions for practical preparation of sub-micrometer xerographic developer.
Liquid toners comprise pigment components and thermoplastic components dispersed in a liquid carrier medium, usually special hydrocarbon liquids. With liquid toners, it has been discovered that the basic printing color (yellow, magenta, cyan, and black) may be applied sequentially to a photoconductor surface, and from there to a sheet of paper or intermediate transfer medium to produce a multi-colored image.
The organic photoconductor products in the market today, generally speaking, are dual layer organic photoconductors (OPCs), which comprise a charge generation layer (CGL) and a charge transport layer (CTL) as key components. In addition to these layers, the photoconductor body can be undercoated or overcoated with other materials to improve adhesion to the substrate or to improve surface wear resistance or to reduce the surface adhesion for improved image transfer efficiency. The OPC with an additional undercoating layer or overcoating layer becomes an organic photoreceptor (OPR) and ready for use in various designs of electrophotographic systems.
Most of the multilayer OPRs in the market are negative charging OPCs in which a thick hole transport hole layer is located on the top of a thin CGL. This is called the standard, or conventional, dual layer OPC. In the conventional case, the CGL usually comprises a photoconductive pigment or dye dispersed in an inert binder, with a pigment/dye content ranging up to about 90 wt %. However, 100% pigment in the CGL is possible where the pigment CGL is vacuum-evaporated in the format of a thin film; see, e.g., U.S. Pat. No. 4,578,334. Besides dispersion stabilizing functions, the CGL binder also plays an important role of adhesion. The conventional dual layer OPC can also be a positive charging OPC when an electron transport is employed.
Positive charging OPCs are also known, either as a single layer structure in which the photoconductive pigment is simply dispersed in a binder matrix or as a thick CGL located on top of the thin CTL. In the case of a positive (+) photoreceptor based on the so-called inverse composite structure, the charge generation elements in the CGL comprise, for example, pigments, dyes, and charge transport molecules. The charge transport layer is based on holes as the carriers.
The conventional dual photoreceptor for electrophotography, in general, exhibits an anomalously high dark decay when the charge generation layer is deposited directly on the metallic substrate, especially aluminum, no matter whether the deposition process is an organic coating process or a vacuum sublimation process. This phenomenon is very prominent when a large contact between the charge generation molecule and substrate metal occurs due to increased amount of charge generation molecule in the CGL or due to the increased thickness of the CGL. This phenomenon happens with most known charge generation molecules, including pigments (poly aromatic pigments including dibromo anthanthrone, perylene, poly azo pigments including bis-, tris-, tetrakis-azo pigments, phthalocyanine pigments, pyrollo-pyrole pigments, and the like), dyes (cyanine dyes, pyrilium dyes, squarylium dyes, and the like), and polymeric charge generation molecules (poly phenylene vinylidene, polyvinyl carbazole, with and without dopant, and the like). This phenomenon is believed to be associated with a charge injection from the substrate metal into the charge generation molecule through the direct contact between these two components; the more contact, the higher the dark decay.
The positive charging photoreceptor for electrophotography, in general, exhibits an increased dark decay and decreased charge acceptance with repeat cycles. This phenomenon is very prominent when a corona-charging system is used and expected to be associated with the positive corona ion charge injected from the surface.
In the case of a positive photoreceptor based on the inverse composite structure, the surface charge injection is believed to involve any or all of the pigment, dye, or hole transport molecule components in the CGL.
Thus, there is a need to minimize the charge injection into the CGL from the metal electrode (conventional dual photoreceptor) or from the free surface (inverse dual photoreceptor).